Re: CD 942 failure tonight

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "John Richardson" <jprich-at-up-dot-net> 

Hi Mark and Bart,

 > The last failure we heard about happened when one string of caps wasn't
 > connected, placing the cap seriously close to a resonant value.  Is it
 > possible this happened?  Is it possible the NST is really, say, 15/120?

It's a 12/120.  I bought two new ones off of e-Bay for $15.00 apiece, and
after posting this guy to the list, I think Harold Weiss bought several
also.  With regards to the strings being hooked up, the end of each string
is screwed into 6" pieces of brass bar stock.  SRSG safety is at .5"
exactly.  NST safety is actually under .5" total.  Center of NST safety, NST
case, and secondary bottom are RF grounded to Cu water pipe that runs thru
the basement, and then about 200 or so feet out to the well in the field.
When taking a video to send to Terry last night, I can see where one of the
caps arced thru it's exterior to a support (Ungrounded!) over 2.5 inches
away.  The bottom of the cap is also split open and melted about 2/3 of the
cap length, and there is positively no place in my system that this could
have arced to.  One of the other caps I sent Terry has zero soot and black
goop, just a nice clean bubble where the foil is becoming visible.  This
bubble is facing towards the next string over, and there is no sign of it
arcing at all.   Every cap and resistor in the bank is first crimped to
uninsulated wire terminals, and then soldered for good measure.  The are
attached to each other with 8/32 machine screws thru holes in fiberglass
electrical project board.  Each string is 9 long, and there are 4 of them.
This should be about .0692.  They are the 942s rated at .15 apiece.  It
should also be noted that all of these failed caps were located at the end
of the string, not in the middle.  These failures were also all on the side
of the cap bank that is connected to one side of the SRSG, not on the side
that goes to the primary.  I don't know if that matters.  It almost seems,
taking into consideration the hot primary wiring, that there is unusually
high amperage in this tank circuit (WHY?).  One other thing that I don't
think I mentioned is that the capacitors were getting warm, not hot, but
warm.  This was on the final run which was certainly under a minute.  I
thought that performance was actually pretty darn good for a first fire,
especially when I could watch it in the dark.  The small toroid and
smallish, poor H/D ratio secondary are the only thing holding me back from
60 + inches, maybe more.  Again that is a seat of the pants assessment.
Can't have caps going off like rifle shots every 60 seconds, though.  I
won't know till I get my meters on the control panel, but is it possible
that this NST is a little more robust than it's rating would imply?

Thanks,
John
 > Just some ideas that haven't already been asked.
 >
 > Mark Broker
 > Chief Engineer, The Geek Group

Thanks,
John

 >
 > On Sun, 29 Feb 2004 16:07:23 -0700, Tesla list <tesla-at-pupman-dot-com> wrote:
 >
 > >Original poster: Bart Anderson <classi6-at-classictesla-dot-com> Hi John,
 > >
 > >Ok, if the rsg total gap is 0.15" or less, then it's not the problem.
 > >Phasing may be a possibility. You are correct, the safety gap should be
 > >firing if the voltage is climbing up (unless it's set too wide). What are
 > >you using for electrodes in the safety gap which is across the rsg?
 > >
 > >The caps rms current rating is excellent "if" circuit resistance is 3
 > >ohms. If it's more like 1 ohm, then your looking at 22 amps or so. The
 > >tempeture rise is then 6.7 per deg C and what you want to keep your eyes
 > >on. I like to see this value down around 2 or less.
 > >
 > >The cap bank voltage rating (18kV) is set near the NST's Vp (17kV), so
 > >there is basically no derating built in - it's on the edge (which many
 > >here claim to get away with and obviously do). When I built my Geek MMC,
I
 > >went with 18 caps per string (36kV). For you to do this and keep the same
 > >capacitance, you would end up with 144 caps (8 strings). If you look at
 > >the cost, large capacitances and derating can get quite expensive, even
 > >for MMC's. The only way to reduce the cost for larger capacitances is to
 > >have individual MMC caps of larger value (maybe someday) or simply take
 > >your chances with less derating built in.
 > >
 > >If you get the same heating with a static gap, then it may be worth
 > >converting the 4 strings of 9 into 2 strings of 18 which drops the tank
 > >capacitance to .033uF. This will allow more primary turns (higher surge
 > >impedance),  a higher cap bank voltage rating (36 kV), and deg C is down
 > >to 1.5 at 1 ohm. It also puts you that much closer to resonance rise
 > >problems if an srsg is used. The .033uF is about the static gap LTR value
 > >and it would be prudent to run it first with a static gap to check
 > >heating. If you then run with an srsg, be aware you may risk the NST as
 > >it's twice as low as the srsg LTR value.
 > >
 > >I'm not sure what the happy medium is, but the wiring and caps heating up
 > >as you have mentioned tells me the rms current is  high and an indication
 > >that circuit impedance is low. That coupled with the low voltage rating
on
 > >the cap bank sounds like a good probability of cap failure.
 > >
 > >
 > >Note, a similarity to consider where the number of caps per string play a
 > >major role:
 > >1) Cap voltage rating.
 > >2) Individual cap temp rise.
 > >
 > >Both need to be considered for MMC's.
 > >
 > >Take care,
 > >Bart
 > >
 > >
 > >Tesla list wrote:
 > >
 > >>Original poster: "John Richardson" <jprich-at-up-dot-net>
 > >>Hi Bart,
 > >>
msnip....